The monitors report Syria is in "contravention" of its government's commitment to withdraw its troops and heavy weapons from population centers, U.N. Secretary-General Ban Ki-moon said in a statement released by his office.

The U.N. observers are tasked with monitoring the implementation of Kofi Annan's six-point peace plan, which calls for President Bashar al-Assad's government and the opposition to end the bloodshed, allow humanitarian groups access to the population, release detainees and start a political dialogue. Annan is the U.N. and Arab League envoy to Syria.

The U.N. Security Council recently authorized sending up to 300 monitors to Syria for 90 days. But as of Wednesday, only 13 were in Syria.

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Syria's information minister said "armed terrorist groups" have committed more than 1,300 violations of a cease-fire since April 12, said the state-run Syrian Arab News Agency, known as SANA. It also reported four Russians will be part of an observer advance team, but that could not be immediately confirmed.

Annan said earlier Syria's foreign minister told him that heavy weapons and troops had been withdrawn from population centers and that military operations had ended, key elements of the peace plan. But reports of shelling and fighting have been dramatic in recent days. Activists say that a military rocket attack Wednesday killed more than 70 people in the city of Hama.

"This is among the deadliest attacks, and is further proof that the Assad regime has no intention of implementing the Annan plan," said Rafif Jouejati, LCC spokeswoman.

Ban is "gravely alarmed by reports of continued violence and killing in Syria, including shelling and explosions in various residential areas as well as armed clashes," the statement said. "He condemns in the strongest terms the continued repression against the Syrian civilian population and violence from any quarter. This situation is unacceptable and must stop immediately."

The secretary-general is "deeply troubled" that weapons, military equipment and troops have not been withdrawn, his office said.

Ban "reminds all concerned parties, particularly the government of Syria, of the need to ensure that conditions for the effective operation of the United Nations military observers are put in place immediately, including a sustained cessation of armed violence."

Arab League ministers, at an emergency meeting in Cairo, called on the United Nations to immediately stop the killings and protect civilians.

Al-Assad's government, as it has done consistently, blamed terrorist groups for the deaths in Hama. The state-run Syrian Arab News Agency said a terrorist group was building a bomb that exploded and killed 16 people, including children.

But activists said the incident, in the Masha'a Altayar neighborhood, was a rocket attack that led to many more deaths when it caused poorly constructed buildings to collapse. Video showed people milling around the rubble. One activist said more than a dozen children were pulled from the wreckage.

The Syrian government's refusal to abide by its commitment is "precisely what we have been concerned about," said Susan Rice, U.S. ambassador to the U.N. "... It is further indication that the government is ready to make commitments and break them just as swiftly, and it certainly casts further doubt -- where there was already a great deal -- on the government's readiness to implement the core elements of the Annan Plan."

The number of Syrian deaths on Thursday rose to 35 people, the LCC said, with many of those deaths in Deir Ezzor, in the east. The dead include four children and two women, the group said.

"It is collective punishment because there are some activists" in that area of Deir Ezzor, said an opposition activist identified as Abu Bilal. "People are trapped in their homes, and the mosques are calling on God for help. The humanitarian situation is bad because we cannot even help our injured. We have no idea if the monitors will visit Deir Ezzor."

Terrorists set off a car bomb that killed a schoolteacher in the city of Aleppo on Thursday, according to SANA. The report said the attackers targeted "national expertise." Syrian authorities say terrorists have been targeting educators, engineers and medical personnel during the crisis.

SANA reported nine people, including army personnel, targeted by "armed terrorist groups" were laid to rest. It said 10 civilians were killed in other locations.

For 13 months, violence has raged between al-Assad's forces and the opposition in a lopsided battle that has seen thousands of civilians killed amid a number of international attempts to broker a peace deal.

The Wednesday incident prompted the opposition Syrian National Council to call for the U.N. Security Council to hold an emergency session to take up the issue of protecting civilians.

The council condemned the international community for continuing to give al-Assad's government time to implement the peace place because it gives "the criminal regime more time to kill."

"The regime is committing all sorts of violations to Annan's plan and ... it has not abided by any of the plan's points," the national council said in a statement denouncing the Hama incident.

The council said it will continue to support the Free Syrian Army, the anti-regime fighter force, to protect "unarmed people," regardless of the future of Annan's plan.

CNN cannot independently verify reports of violence and deaths within Syria, as the government has restricted access by international media.

The United Nations estimates at least 9,000 people have since died in the conflict, while activist groups put the death toll at more than 11,000.

William Hague, Britain's foreign minister, had a Twitter message for al-Assad after an international tribunal found former Liberian President Charles Taylor guilty of aiding and abetting war crimes in neighboring Sierra Leone's notoriously brutal civil war.

"#CharlesTaylor: Justice has been done. Remember his victims, & remind #Assad: there is no expiry date for crimes against the innocent," Hague said.